In a base station having an adaptive antenna array, when a subscriber device is due to exchange communications with its base station, a beam from the base station is directed towards that subscriber device (referred to herein as “beamforming”), and the received power for that subscriber device is respectively increased, roughly by 10*log 10 (Number of Antennas) relative a non beam formed case. Modern cellular wireless systems typical employ multi-user scheduling in order to improve the overall system throughput. This is done by scheduling transmissions to/from a given user when its channel power peaks, while scheduling-transmissions to/from other users when the channel power of that given user fades. The overall effect is that users are scheduled for transmissions under high signal to noise ratio conditions. Therefore, the additional power increase achievable by adaptive antenna system yields limited improvement in terms of throughput. Furthermore, one other problem characterizing this kind of a solution is the inherent delay associated with the transmissions, because as was explained above, transmissions are exchanged only when there is a peak in the channel power.
The solution that has been proposed to these problems was in the form of working in a Spatial Division Multiplexing Access (“SDMA”) type of radio network, where a number of channels are transmitted simultaneously to a number of users, in order to reduce that delay and increase the overall (spectral) efficiency and throughput. Therefore in SDMA radio networks the same time-frequency resources are used multiple times thereby increasing the throughput and spectral efficiency of the network when compared to the case where SDMA is not employed.
Still, this method also suffers from several drawbacks. One of these drawbacks is related to the transmission of a control channel by the base station and its reception by the subscriber stations. All subscribers' devices must be provided with the appropriate information to allow each one of them to know who are the subscriber(s) due to receive transmissions in the next frame. For OFDM or OFDMA based technologies this information is referred to as a “map” and would typically contain time and frequency allocation assigned to the different subscribers. The information contained in the map is required for a number of reasons, for example a subscriber that tries to demodulate a time and frequency allocation beamformed to another user, will typically fail. Moreover, the map cannot be beamformed since all subscribers need to be able to read it. In other words, currently there is no way to beamform or spatial multiplex the maps to allow improving the spectral efficiency of the maps.
Another way to increase spectral efficiency is to increase the number of sectors per site, e.g. have 6 or more sectors per site. However the increase in this case relies on the fact that all sectors must be occupied. However as we add more sectors, some sectors are likely to have little or no traffic, so that in this case spectral efficiency gain would not be achieved in a practical setting. In addition this solution does not increase the peak rate of a user or the peak throughput of a sector, in contrast these two parameters are lower as interference from other sectors is increased.
The Spatial Division Multiplexing Access (“SDMA”) technique referred to herein is a technique known in the art, and also used in advanced cellular standards e.g. IEEE Std 802.16e-2005 (WiMax).